Power plants generate electricity by burning coal in the furnaces of boilers, which are used to generate steam for turbines that turn generators to produce electricity. A byproduct of burning coal is coal ash. During such coal burning operations in the furnaces of boilers, coal ash is produced in two forms. First, there are light particles, known as fly ash, which leave the boiler with the flue gases and are later trapped by precipitators. Secondly, there are heavier particles, called bottom ash, that remain in the boilers' furnace after combustion and are later removed from the furnaces. In general, both fly and bottom ash are referred to as ash. Such ash can contain a number of toxic materials depending on the type of coal being used. For example, the ash may include one or any combination of mercury, molybdenum, selenium, strontium, thallium, arsenic, dioxins, beryllium, boron, cadmium, chromium, hexavalent chromium, cobalt, lead, manganese, vanadium, and polyaromatic hydrocarbons.
This byproduct of generating electricity, coal ash, is normally deposited in large ash basins, or landfills, that are typically a depression in the ground, either naturally occurring or man-made near the power plant itself. When this practice of storing coal ash started, at the dawn of the power industry, there was no concern for the contaminating aspects of coal ash either to the soil itself, or to the groundwater below. Over the course of decades, the accumulation of millions of tons of ash has occurred near power plants throughout the country, in what can be described as unlined disposal sites.
Because power plants require large quantities of cooling water to condense the steam back into water for reuse in a power generation cycle of the power plant, most power plants are located near a river, lake or other body of water. The close proximity of a power plant to a body of water means the water table, in the general area of the power plant, is relatively close to the ground surface. Thus, the close proximity of the power plant to a body of water can result in many of the unlined ash basins to be partially located in the water table. Rainwater, water from the water table itself, and water from power plant operations used in some cases to carry the ash to the ash basin can be mixed in the unlined ash basin to create a thick layer of wet ash covered, in some cases, by many feet of water contaminated with coal ash both in suspension and in diluted forms. The contaminated water can enter into the water table because most ash basins are unlined or have no barrier to prevent the contaminated water or other contaminants from the coal ash from entering into the water table. Further, the contaminated water or other contaminants from the ash can move into an adjacent body of water once in the water table.
As science advanced, and determined the detrimental effects of ash there has been a public outcry, coupled with regulatory requirements, to remedy the situation of ash basins contaminating the groundwater table and adjacent bodies of water. This public outcry has resulted in strong regulatory requirements to clean up such unlined disposal sites to stop the contamination of groundwater Similar situations have also occurred with current waste landfills and current hazardous waste dumps. Many solutions have been attempted to address the challenges presented in cleaning up an unlined disposal site.
One current solution for cleaning up unlined ash basins is the excavation and transportation, by truck or rail, of the ash to a lined landfill in a location away from the power plant where the water table is lower. This solution presents several problems of its own including the need to acquire a large tract of land suitable for a landfill, obtaining the necessary regulatory permits, obtaining the agreement of the neighbors in the area, the large cost of associated with the transportation itself, and the risk of a traffic accident during the transport that will cause a large ash spill contaminating a neighborhood, its soil, and its groundwater.
Another potential solution is the construction of a new lined ash basin at the power plant site itself. The problems with this approach are finding a suitable large amount of space within the boundaries of the power plant itself to build the new landfill, a new disruption of the water table, and the risks and costs associated with moving the large quantities of ash involved. These problems are in addition to obtaining a permit from regulatory bodies to be allowed to build a new landfill within the water table.
There is a proposal to simply cap the ash basin or landfill in place. Capping the ash basin will eliminate the addition of rainwater to the ash basin, which may cause contaminated water to go into the ground water. However, capping does not eliminate the flow of groundwater through the body of the ash basin such that contamination of the groundwater and nearby bodies of water will continue. Further, most regulatory bodies will not accept capping as a solution.
Another proposal is to push the ash from one end of the ash basin over on top of an adjacent area to reach the bottom of the basin so as to expose native soil and allow a liner to be installed. The problem with this solution is that coal ash should be kept wet to keep airborne particulates down when working with the ash. The wet ash can be more like a liquid mud and behave as a slurry such that that the ash will not remain piled up for very long thus creating a never ending process of piling up the ash, while it continues to fall back. The ash should not be allowed to dry too much or airborne particulates may become an issue when working with the ash.
Another possible solution is to stabilize the ash into a less volatile form. One such technique is crystalizing the ashes using an electric discharge like a plasma torch. The problem with this technique is the enormous amounts of energy required to achieve the crystallization state, and then the crystals must be transported, requiring more energy to be spent, to a landfill with sufficient capacity and approved to receive such crystalized ash. Another technique is to use the ash in industrial applications like in cement mixes and for the manufacturing of sheet rock. Unfortunately, while some of these applications are viable, there is not enough demand for the quantities of ash being produced by power plants and there are still handling and transportation issues with the ash.